Chemical contamination of the environment, particularly of soil and groundwater is currently a widespread problem that is prevalent in many parts of the industrialized world. Industrial pollution has contaminated millions of acres of soil and associated aquifers. Often, cleanup of the contamination is avoided because of the costs of remediation, and the land may remain unused or abandoned.
Typical remediation (decontamination) strategies include incineration and/or removal. In the case of contamination of large areas, an on-site incinerator may be warranted. In other cases, generally on a smaller scale, excavation and removal to an RCRA (Resource Conservation & Recovery Act) compliant incinerator or landfill may be employed. Both of these methods require intensive labor and mechanical effort resulting in high costs. Other methods in frequent use include pumping and treating, vacuum extraction, steam flooding, air sparging and soil flushing.
Treatment of contaminated soils in situ by various methods has been pursued because it does not require excavation or hauling and is less costly. Oxidation is one technique used to treat contaminated soils in situ. For example, one such oxidation technique that has been widely used is oxidation of organic compounds with ozone, potassium permanganate or hydrogen peroxide.
Additionally, efforts have been made to reduce the costs of treatment with chemicals by more effectively directing the chemicals used in the treatment to the contaminated soil to the appropriate location and depth. For example, U.S. Pat. No. 5,525,008 to Wilson discloses a method of directing the flow of the oxidizing treatment solutions to contaminants in the soil or groundwater. The invention discloses the use of multiple horizontally spaced sealed injection wells, with the goal of directing the reactive solution through the contaminated area.
Another approach at directing treatment chemicals to the contaminates in soil is disclosed by U.S. Pat. No. 4,591,443 to Brown. The '443 patent discloses a method of decontaminating subterranean soil by controlling the mobility of aqueous treatment fluids. In order to direct the treatment fluid containing the active chemical, such as hydrogen peroxide to the intended reaction site, hydratable polymers are used as viscosity modifiers. Optionally, cross linking agents may be added to further increase the viscosity of the treatment fluid. Surfactants are employed to decrease the interaction of metals or clays with peroxide. The '443 patent further discloses the use of peroxide stabilizers, free radical initiators such as iron (Fe) and also free radical inhibitors. Penetrating pre-treatment fluids for altering the reactivity of the soil or rock formation and inactivating hydrogen peroxide decomposition catalysts are also disclosed.
The '443 patent teaches that these various oxidation and flow modifiers combine to provide a degree of spatial and temporal control of the oxidizing treatment chemicals, with the stated intention of reacting with the desired chemical contaminants rather than the surrounding naturally occurring minerals and soil. The '443 patent does not address the use of bioremediation, to treat many common forms of contamination, particularly organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), thereby permitting reduced chemical exposure and loading of the site. The '433 method would be expected to produce poor oxidation of, e.g. anthracene and chrysene, and other such hydrocarbons.
Bioremediation has begun to gain wider acceptance as a viable treatment technology for remediating soils, sediments and subsurface sites contaminated with hydrocarbons. The attractiveness of bioremediation arises at least in part from the fact that the process takes advantage of intrinsic biodegradative processes of microorganisms and because the compounds that are the target of remediation are degraded to innocuous end products. In this respect, bioremediation-based remediation approaches using either in situ or off site designs have been successfully employed for remediation of soils and subsurface sites contaminated with lighter fractions of petroleum or petroleum products, and for the lower molecular weight and more water soluble aromatic components of petroleum products, represented, for example, by benzene, toluene, ethylbenzene and xylenes.
Bioremediation strategies, however, often have limited applicability when soils, sediments and subsurface sites are contaminated with complex mixtures of highly hydrophobic aromatic compounds such as commonly occurs for instance with tar residues. The polycyclic aromatic hydrocarbons (PAHs) that are component of tar residuals, remain a challenge for the application of in situ remediation strategies owing to the low aqueous solubility of mixed PAH components. Such PAHs are produced, for example, from the volatile components of bituminous coals in coal carbonization, from the residue of gasifying oils in oil gas processes, and from the cracking of enriching oils in carbureted water gas production at former manufactured gas plant (MGP) sites. Bioremediation of such PAH-contaminated sites is hampered by the low aqueous solubility of PAH compounds, which leads to low bioavailability where the compounds are not available for microbial action that depends on aqueous chemistry and enzyme action.
Hydrogen peroxide, in the presence of ferrous ions (Fe++) as a catalyst, generates a strong nonspecific oxidant hydroxyl radical that reacts with most organic compounds at diffusion-controlled rates of 107 to 1010 M−1 sec−1. This is known as Fenton's reaction and has been used for the destruction of organic contaminants including (poly)chlorinated aromatic compounds and a variety of herbicides in aqueous solutions or soils. However, little evidence is available regarding whether the Fenton's reaction can mineralize organic contaminants, or whether the resulting partially oxidized organic compounds pose less hazards than the parent compounds. Moreover, use of Fenton's reaction produces soil pH changes which are incompatible with bacteria and make subsequent use of bioremediation methods ineffective.
Although Fenton's reagent has the potential to non-specifically oxidize many PAHs, it also results in a substantial lowering of the soil pH, e.g. to a pH of between 2 and 3. At these pH levels, many heavy metal contaminates become solubilized and migrate into ground water. Moreover, this pH range is subsequently incompatible with many forms of biological treatment.
As noted above, one serious disadvantage of the use of chemical oxidation with bioremediation is the lowering of soil pH to levels which solubilize many heavy metals and which are unacceptable for sustaining many useful bacteria. Nonetheless, various attempts at treating contaminated soils have included a combination of bioremediation and chemical treatment.
U.S. Pat. No. 5,955,350 to Soni et al. discloses the stepwise use of biological treatment, then chemical treatment followed by another biological treatment of organic waste. Hydrogen peroxide is a strong oxidant and is very reactive. The '350 patent discloses the use of Fenton's reagent and peroxide between two stages of biological remediation.
One severe drawback of the use of Fenton's reagent is that in some circumstances, the rapid reaction of peroxide can result in excessive heat and consequent generation of steam, creating high pressures and potentially resulting in an explosive release. In the field, various approaches to the problem of explosive potential are used. These include adaptations such as venting the formation or utilizing a slow introduction of the peroxide. The '350 patent discloses a slow rate of addition of peroxide in order to avoid high rates of oxidation. Manageable temperatures are maintained by slow addition of hydrogen peroxide, exemplified by the addition of an approximate rate of 1 to 100 mg hydrogen peroxide per hour per gram of contaminated soil to the ferrous salt solution.
U.S. Pat. No. 5,610,065 to Kelley et al. also discloses combined chemical and biological remediation including the use of Fenton's reagent for degradation of high molecular weight PAHs in soil. The '065 patent is silent as to pH control during the oxidation process. It is not surprising, therefore, that the '065 patent follows chemical oxidation with additional microbial inoculation in an effort to restock the microorganism population after exposure to the harsh oxidation conditions. The '065 patent also discloses the use of a lower alcohol to increase the aqueous solubility of PAHs.
U.S. Pat. No. 5,741,427 to Watts utilizes stabilizers to provide chemical ligands for Fe(III) species during Fenton's oxidation. The stabilizer ligands are provided by phosphates, silicates, or citrates. Control of pH is not addressed. Similarly, the aforementioned problems associated with oxidation of soil contaminants are not addressed.
To date the known methods which employ oxidation and/or bioremediation techniques to treat contaminated soils all suffer from the various disadvantages discussed above. It would therefore be desirable to provide a means by which oxidation of contaminates can occur efficiently without the need for undesirable changes in pH. There is a need for a remediation method that is operational in a neutral pH range with increased biocompatibility and that reduces the solubilization of heavy metals. It would also be desirable to provide a treatment method which avoids further contamination due to the formation of unwanted or toxic by-products as a result of the treatment.